The invention relates to an apparatus for detecting the position of an object which can move along a predefined path. Such apparatus may be used for controlling the position of a lift cage or the like and thus ensuring its positioning at predetermined stops according to floor levels.
European patent 0 694 792 discloses an apparatus of this type for detecting the position of a movable object, which has an acoustic signal waveguide which extends along a travel path and has a predetermined, uniform speed of propagation of sound, and has a signal input coupler, which is connected to a signal generator and is located on the movable object, to couple an acoustic signal into the acoustic signal waveguide. In this case, signal output couplers are arranged at both ends of the acoustic signal waveguide and are each connected to a counter, the two counters being clocked by a clock generator and connected to a subtracter for the output signals from the two counters. The output signal from the subtracter, as a measure of the propagation-time difference of the acoustic signal coupled in from the point at which it is coupled in to the signal output couplers, can be processed by an evaluation unit to form a signal which is representative of the instantaneous position of the movable object on the travel path, the signal input coupler operating with a signal spacing which is greater than the propagation time of sound from one end of the travel path to the other.
By means of the evaluation unit and a computing algorithm implemented in it, the measured propagation times from the sound input coupler to the sound output couplers are assigned a position value. Particularly in lift construction, the evaluation unit is calibrated by the flush level point of each story being assigned the result of the computing algorithm at this point, and the lift position being associated with this numerical value. Because of temperature-induced changes in the building and/or acoustic signal waveguide length, or as a result of a change in the speed of sound in the acoustic signal waveguide arising from temperature and diffusion effects, however, the assignment between the flush level point and the numerical value associated by the calibration is shifted.
German patent 3 608 384 further discloses the principle, in measuring distance travelled by means of determining the propagation time of acoustic pulses, to provide a calibration section alongside the measurement section, in order to be able to convert the propagation-time measurement results into a travel length which is as accurate as possible. However, in this case the measurement and calibration sections are physically separated from each other, in order that the actual measurement can be carried out simultaneously with the calibration measurement.
It is an object of the invention to provide a apparatus for detecting the position of an object which can be moved along a prescribed path, with which it is possible to move simply and accurately to predetermined positions of the movable object on a travel path of great lengths.
Further objects, embodiments and advantages of the invention will become apparent from the following description and the claims.
An apparatus for detecting the position of an object moveable along a predetermined travel pathin accordance with the present invention comprises
an acoustic signal waveguide extending along the travel path and having a predetermined, uniform speed of propagation of sound;
a signal input coupler located on the moveable object, to couple a clocked acoustic signal into the acoustic signal waveguide,
signal output couplers at both ends of the acoustic signal waveguide and each being connected to an evaluation unit for determining the propagation-time difference of the sound signal from a position at which it is coupled in to the signal output couplers and for generating a signal representative of the instantaneous position of the moveable object on the travel path,
wherein a calibration signal input coupler connected to a calibration acoustic signal generator for a calibration acoustic signal is provided on one of the signal output couplers, the evaluation unit calculating a correction variable for subsequent determinations of position from the calibration path passed through by the calibration acoustic signal to the other signal output coupler and the propagation time of the calibration acoustic signal over the calibration path.